The Harvey and Kirsten murine sarcoma viruses (HaSV and KiSV) carry related src genes, each of which is a representative of a small family of conserved cellular sarc genes. The src gene of each virus endcodes a protein of approximately 21,000 daltons (p21) which can induce malignant transformation in infected cells. The HaSV and KiSV src proteins are related immunologically, both to each other and to the endogenous p21 species expressed in a variety of vertebrate cells. In order to facilitate the identification and functional characterization of different p21 species, we are preparing potent and specific monoclonal antibody reagents directed against these proteins. We find that some monoclonal antibodies to the HaSV src protein bind only to p21 species encoded by the most closely related genes, while others bind to more divergent species as well. These reagents should therefore permit purification of specific forms of p21. We are particularly interested in expression of p21 in hematopoietic cells, and hope to use the antibodies to study the action of p21 in transforming erythroid cells. We also wish to study the potential role of particular p21 species in the control of normal hematopoietic growth and cell differentiation.